Use of ion beam tails to manufacture a workpiece

ABSTRACT

One method of implanting a workpiece involves implanting the workpiece with an n-type dopant in a first region with center and a periphery. The workpiece also is implanted with a p-type dopant in a second region complementary to the first region. This second region also has a center and a periphery. The periphery of the first region and the periphery of the second region at least partially overlap. A dose at the periphery of the first region or second region is less than a dose at the center of the first region or second region. The region of overlap may function as a junction where charge carriers cannot pass.

This application is a divisional application of and claims priority toU.S. Non-Provisional application Ser. No. 13/178,118, filed on Jul. 7,2011, which is incorporated by reference herein in its entirety.

FIELD

This invention relates to ion implantation and, more particularly, toion implantation of solar cells.

BACKGROUND

Ion implantation is a standard technique for introducingconductivity-altering impurities into a workpiece. A desired impuritymaterial is ionized in an ion source, the ions are accelerated to forman ion beam of prescribed energy, and the ion beam is directed at thesurface of the workpiece. The energetic ions in the ion beam penetrateinto the bulk of the workpiece material and are embedded into thecrystalline lattice of the workpiece material to form a region ofdesired conductivity.

Solar cells are one example of a device that uses silicon workpieces.Any reduced cost to the production of high-performance solar cells orany efficiency improvement to high-performance solar cells would have apositive impact on the implementation of solar cells worldwide. Thiswill enable the wider availability of a clean energy technology.

Solar cells typically consist of a p-n semiconducting junction. FIG. 1is a cross-sectional view of an interdigitated back contact (IBC) solarcell. In the IBC solar cell 205, the junction is on the back ornon-illuminated surface. In this particular embodiment, the IBC solarcell 205 has an n-type base 206, an n+ front surface field 207, apassivating layer 208, and an anti-reflective coating (ARC) 209. Thepassivating layer 208 may be SiO₂ and the ARC 209 may be SiN_(x) in oneinstance, though other materials or dielectrics may be used. Photons 214enter the IBC solar cell 205 through the top (or illuminated) surface,as signified by the arrows. These photons 214 pass through the ARC 209,which is designed to minimize the number of photons 214 that arereflected away from the IBC solar cell 205. The photons 214 enterthrough the n+ front surface field 207. The photons 214 with sufficientenergy (above the bandgap of the semiconductor) are able to promote anelectron within the valence band of the semiconductor material to theconduction band. Associated with this free electron is a correspondingpositively charged hole in the valence band.

On the back side of the IBC solar cell 205 is an emitter region 215. Thedoping pattern of the emitter region 215 is alternating p-type andn-type dopant regions in this particular embodiment. The n+ back surfacefield 204 may be approximately 450 μm in width and doped with phosphorusor other n-type dopants. The p+ emitter 203 may be approximately 1450 μmin width and doped with boron or other p-type dopants. This doping mayenable the junction in the IBC solar cell 205 to function or haveincreased efficiency. This IBC solar cell 205 also includes apassivating layer 212, n-type contact fingers 210, p-type contactfingers 211, and contact holes 213 through the passivating layer 212.

To form the IBC solar cell 205, at least two patterned doping steps maybe required. If the p+ emitter 203 and n+ back surface field 204 overlapafter these patterned doping steps and the overlap region has highdopant concentrations for both n-type and p-type dopants, there will bea very narrow depletion region between the p+ emitter 203 and n+ backsurface field 204. This means that shunting between the p+ emitter 203and n+ back surface field 204 can occur. High dopant concentrationsbetween 1E18 cm⁻² to 1E19 cm⁻² or around the mid E19 cm⁻² may lead toshunting.

To avoid such shunting, tight alignment of the p+ emitter 203 and n+back surface field 204 is required so that no such overlap occurs.However, even if no overlap occurs, if the p+ emitter 203 and the n+back surface field 204 touch then charge carriers can cross between thep+ emitter 203 and the n+ back surface field 204 using quantumtunneling. In such a case, the space charge region between the p+emitter 203 and the n+ back surface field 204 will be shallow, enablingthe quantum tunneling. Since the p+ emitter 203 and the n+ back surfacefield 204 are located on the same side of the IBC solar cell 205 in theemitter region 215, such quantum tunneling also can shunt any junctionbetween the p+ emitter 203 and the n+ back surface field 204.

The IBC solar cell 205 may be improved by increasing the distancebetween the p+ emitter 203 and the n+ back surface field 204 to, forexample, approximately 1 μm. However, maintaining alignment of the twopatterned doping steps to keep such a distance while ensuring the p+emitter 203 and the n+ back surface field 204 are not close enough forquantum tunneling is difficult. Increasing this distance between the p+emitter 203 and the n+ back surface field 204 to larger dimensions alsohas problems. If large undoped regions exist between the p+ emitter 203and the n+ back surface field 204, then charge carriers will not berepelled from the surface of the IBC solar cell 205. Unless this surfaceis well-passivated, recombination can occur at the surface of thisundoped region. Recombination degrades voltage and current of the IBCsolar cell 205. Therefore, there is a need in the art for an improvedmethod of doping solar cells and, more particularly, an improved methodof doping IBC solar cells using ion implantation.

SUMMARY

According to a first aspect of the invention, a method of implanting aworkpiece is provided. The method comprises implanting the workiece withan n-type dopant in a first region. This first region has a center and aperiphery. The workpiece is implanted with a p-type dopant in a secondregion complementary to the first region. This second region has acenter and a periphery. The periphery of the first region and theperiphery of the second region at least partially overlap. A dose at theperiphery of the first region is less than a dose at the center of thefirst region and a dose at the periphery of the second region is lessthan a dose at the center of the second region.

According to a second aspect of the invention, a method of implanting aworkpiece is provided. The method comprises implanting an entire surfaceof the workpiece with a first dopant. The workpiece is implanted with asecond dopant in a second region that is a fraction of the surface. Thesecond region has a center and a periphery and has a larger dose at thecenter than at the periphery.

According to a third aspect of the invention, a solar cell is provided.The solar cell comprises a workpiece having a first surface. A pluralityof n-type regions are in the first surface. Each of the n-type regionshas a center and a periphery. The center of the n-type region has a doselarger than in the periphery of the n-type region. A plurality of p-typeregions are in the first surface complementary to the plurality of then-type regions. Each of the p-type regions has a center and a periphery.The center of the p-type region has a dose larger than in the peripheryof the p-type region. The periphery of the p-type region and theperiphery of the n-type region at least partially overlap.

According to a fourth aspect of the invention, a solar cell is provided.The solar cell comprises a workpiece having a first surface. This firstsurface is doped to a first conductivity to a first depth. A pluralityof second regions are in the first surface. Each of the second regionsis doped to a second conductivity opposite the first conductivity andeach of the second regions has a center and a periphery. The center ofthe second region has a dose larger than in the periphery of the secondregion. A space charge region is between the plurality of the secondregions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present disclosure, reference is madeto the accompanying drawings, which are incorporated herein by referenceand in which:

FIG. 1 is a cross-sectional view of an IBC solar cell;

FIG. 2 is a diagram illustrating a first embodiment of a dose profile;

FIG. 3 is a diagram illustrating a second embodiment of a dose profile;

FIG. 4 is a diagram illustrating a third embodiment of a dose profile;

FIGS. 5A-B are cross-sectional side views of a first embodiment ofimplanting a workpiece;

FIGS. 6A-B are cross-sectional side views of a second embodiment ofimplanting a workpiece;

FIG. 7 is a cross-sectional side view of a third embodiment ofimplanting a workpiece; and

FIG. 8 is a cross-sectional side view of implanting through a mask.

DETAILED DESCRIPTION

The embodiments of this method are described herein in connection withan ion implanter. Beamline ion implanters, plasma doping ion implanters,focused plasma systems, systems that modulate a plasma sheath, or floodion implanters may be used. However, gaseous diffusion, furnacediffusion, laser doping, other plasma processing tools, or other methodsknown to those skilled in the art also may be used for the blanket orselective implant or doping steps. While specific n-type and p-typedopants are listed, other n-type or p-type dopants may be used insteadand the embodiments herein are not limited solely to the dopants listed.Furthermore, while one particular embodiment of a solar cell isspecifically listed, embodiments of this process may be applied to othersolar cell designs or even other workpieces such as semiconductorwafers, light-emitting diodes (LEDs), or flat panels. And while referredto as first or second, the steps in the embodiments disclosed herein mayoccur in any order and are not limited to the exact sequence listed.This nomenclature is for explanation only. Thus, the invention is notlimited to the specific embodiments described below.

FIG. 2 is a diagram illustrating a first embodiment of a dose profile. Aperfectly parallel ion beam that is implanted through a shadow mask orstencil mask will produce an implanted region in a workpiece with anabrupt transition between implanted and unimplanted regions. This shadowmask or stencil mask may sit on the workpiece or be disposed upstream ofthe workpiece. FIG. 2 illustrates such a dose profile in the workpieceafter implantation with a perfectly parallel ion beam that has verysharp or distinct edges of regions that have been implanted.

FIG. 3 is a diagram illustrating a second embodiment of a dose profile.Any ion beam has an inherent spread of angles, such as those due tospace charge effect or ion beam “blow up.” This spread of angles meansthe transition between the implanted and unimplanted regions will not beperfectly abrupt as seen in FIG. 2. Instead, the transition in FIG. 3reflects the distribution of angles and the geometry of the shadow maskor stencil mask and the workpiece. Thus, shoulders 100 and tails 101 areformed in the dose profile. If the distance between the back of theshadow or stencil mask and the workpiece is reduced, then the tails 101will shrink. If the distance between the front of the shadow or stencilmask and the workpiece is reduced, then the shoulders 100 will shrink.Both will make the dose profile of FIG. 3 resemble the dose profile ofFIG. 2 more closely.

The spread of angles of the ion beam and geometry of the shadow orstencil mask and the workpiece can be used to produce a tail 101 thatenables light doping of the gap between n-type regions and p-typeregions of a workpiece. This may be applied to solar cells, such as theIBC solar cell 205 of FIG. 1.

FIG. 4 is a diagram illustrating a third embodiment of a dose profile.In this embodiment, a region 103 is of a first conductivity and theregions 104 (represented by the dotted lines) are of a secondconductivity. These conductivities may be opposite and can representn-type and p-type doped regions. There also are regions 105 where theregion 103 and region 104 overlap. These regions 105 can serve as ajunction where charge carriers cannot pass. The junction will occurwhere equal amounts of dopant of the first conductivity and secondconductivity are present. The tails of the ion beams are used lightlydope the regions 105 between the region 103 and regions 104. In a solarcell, such as the IBC solar cell 205, this light doping provides anelectric field that inhibits charge carriers from moving to the surfaceand recombining. The doping level can be configured to be low enough toprevent or reduce quantum tunneling.

FIGS. 5A-B are cross-sectional side views of a first embodiment ofimplanting a workpiece. In FIG. 5A, the workpiece 300 is implanted witha first species 304. The workpiece 300 may be silicon and may be used tomanufacture a solar cell. The first species 304 is implanted through theaperture 303 in the mask 301. This mask 301 may be a shadow mask orstencil mask and in this embodiment is illustrated upstream of theworkpiece 300. The mask 301 blocks the first species 304 from implantingthe workpiece 300 except through the aperture 303.

The first species 304 forms the first region 305 in the workpiece 300.The first region 305 has a center and a periphery at each end of thedimension 306. The dimension 306 of the first region 305 is slightlylarger than the dimension 307 of the aperture 303. This is partly due tothe space charge of the first species 304 in the ion beam.

In FIG. 5B, the workpiece 300 is implanted with a second species 310.The second species 310 is implanted through an aperture 309 in the mask308. This mask 308 may be a shadow mask or stencil mask and in thisembodiment is illustrated upstream of the workpiece 300. The mask 308blocks the second species 310 from implanting the workpiece 300 exceptthrough the aperture 309. The first species 304 and second species 310have opposite conductivities in one embodiment, such that one is n-typeand the other is p-type.

The second species 310 forms the second region 311 in the workpiece 300.The second region 311 has a center and a periphery at each end of thedimension 312. The dimension 312 of the second region 311 is slightlylarger than the dimension 313 of the aperture 309. This is partly due tothe space charge of the second species 310 in the ion beam. The secondregion 311 and first region 305 in this embodiment are complementary oraligned with respect to one another.

The first region 305 and second region 311 both may have a dose gradientor profile such that each has a larger dose in the center than at theperiphery. The periphery of the first region 305 and periphery of thesecond region 311 overlap in FIG. 5B. This overlap region 315, which isshaded in FIG. 5B, contains both n-type and p-type species. A junctionis formed at the location where the concentrations of the n-type andp-type species are the same. A depletion region also is formed and thedepth of this depletion region increases as the dopant concentration inthe overlap region 315 decreases. When the junction is formed at asufficiently low dopant concentration, the depletion region will be deepenough to prevent quantum tunneling of charge carriers.

In one instance the first region 305 and second region 311 of FIG. 5Bmay be the p+ emitter 203 and the n+ back surface field 204 of the IBCsolar cell 205 of FIG. 1, though other designs or embodiments arepossible. While only one first region 305 and one second region 311 areillustrated in FIG. 5B, multiple first regions 305 and second regions311 may be used across a surface of the workpiece 300. Tails, such asthose illustrated in FIG. 3, may be used or adjusted to form thejunction or overlap region 315.

FIGS. 6A-B are cross-sectional side views of a second embodiment ofimplanting a workpiece. In FIG. 6A, the workpiece 300 is implanted witha first species 316. The workpiece 300 may be silicon and may be used tomanufacture a solar cell. The first species 316 forms the first region317 in the workpiece 300. The implant of the first species 316 may be ablanket implant of the entire surface of the workpiece 300 in oneinstance and may be uniform.

In FIG. 6B, the workpiece 300 is implanted with a second species 320.The second species 320 is implanted through an aperture 319 in the mask318. This mask 318 may be a shadow mask or stencil mask and in thisembodiment is illustrated upstream of the workpiece 300. The mask 318blocks the second species 320 from implanting the workpiece 300 exceptthrough the aperture 319. The first species 316 and second species 320have opposite conductivities in one embodiment, such that one is n-typeand the other is p-type.

The second species 320 forms the second region 321 in the workpiece 300.The second region 321 is only a fraction of the surface of the workpiece300 in this embodiment. The second region 321 has a center and aperiphery at each end of the dimension 322. The dimension 322 of thesecond region 321 is slightly larger than the dimension 323 of theaperture 319. This is partly due to the space charge of the secondspecies 320 in the ion beam. While the second region 321 is illustratedas implanted deeper than the first region 317, the second region 321also may be shallower or approximately the same depth as the firstregion 317.

The second region 321 may have a dose gradient or profile such that ithas a larger dose in the center than at the periphery. This larger doseof the second region 321 or within the second region 321 may be largerthan the dose of the first region 317. Thus, counterdoping may occur. Ateach end of the periphery of the second region 321 is a space chargeregion 324. A dopant profile in the periphery of the second region 321or first region 317 is configured to prevent shunting across this spacecharge region 324. In one instance the first region 317 and secondregion 321 of FIG. 6B may be the p+ emitter 203 and the n+ back surfacefield 204 of the IBC solar cell 205 of FIG. 1, though other designs orembodiments are possible. While only one second region 321 isillustrated in FIG. 6B, multiple second regions 321 may be used across asurface of the workpiece 300. Tails, such as those illustrated in FIG.3, may be used or adjusted to form the second region 321 or space chargeregions 324.

FIG. 7 is a cross-sectional side view of a third embodiment ofimplanting a workpiece. In one embodiment, FIG. 7 is an additional stepto the embodiment of FIGS. 6A-B. In FIG. 7, the workpiece 300 isimplanted with a third species 327. The third species 327 may be thesame as the first species 316 of FIG. 6A in one instance. The thirdspecies 327 is implanted through apertures 326 in the mask 325. Thismask 325 may be a shadow mask or stencil mask and in this embodiment isillustrated upstream of the workpiece 300. The mask 325 blocks the thirdspecies 327 from implanting the workpiece 300 except through theapertures 326. The third species 327 and second species 320 haveopposite conductivities in one embodiment, such that one is n-type andthe other is p-type.

The third species 327 forms the third regions 328 in the workpiece 300.The third regions 328 are only a fraction of the surface of theworkpiece 300 in this embodiment. The third regions 328 each have acenter and a periphery at each end of the dimension 329. The dimension329 of the third regions 328 is slightly larger than the dimension 330of the apertures 326. This is partly due to the space charge of thethird species 327 in the ion beam. While the third regions 328 areillustrated implanted deeper than the first region 317 (from the surfaceof the workpiece 300 to the dashed line), the third regions 328 also maybe shallower or approximately the same depth as the first region 317.The third regions 328 may be implanted to the approximately same depthor a different depth than the second region 321.

The second region 321 and third region 328 both may have a dose gradientor profile such that each has a larger dose in the center than at theperiphery. The periphery of the second region 321 and periphery of thethird regions 328 overlap in FIG. 7. The overlap regions 331, which areshaded in FIG. 7, contain both n-type and p-type species and form ajunction. In one instance the second region 321 and third region 328 ofFIG. 7 may be the p+ emitter 203 and the n+ back surface field 204 ofthe IBC solar cell 205 of FIG. 1, though other designs or embodimentsare possible. While only one second region 321 is illustrated in FIG. 7,multiple second regions 321 and third regions 328 may be used across asurface of the workpiece 300.

While a certain number of regions were illustrated in FIGS. 5A-B, 6A-B,and 7, this is for simplicity. Some workpieces have many more implantedregions across a surface. For example, these implanted regions may bebetween approximately 450 μm to 1450 μm. Thus, the embodimentsillustrated may be repeated across some or all of a surface of aworkpiece. In one embodiment, a solar cell contains a plurality ofimplanted regions across an entire surface. Furthermore, the dosegradient or profile disclosed in FIGS. 5A-B, 6A-B, and 7 may vary. Forexample, the dose may continuously decrease from the center to theperiphery. Of course, other dose gradients or profiles are possible. Inone particular embodiment, the center has a dose of 1E15 cm⁻² to 3E15cm⁻² and the periphery has a dose of 1E13 cm⁻² to 1E14 cm⁻². Differentdoses in the center or periphery are possible if shunts are prevented.

There are many ways to achieve or adjust a lightly-doped tail orperiphery of an implanted region. FIG. 8 is a cross-sectional side viewof implanting through a mask. In this embodiment, the species 402 isimplanted through the aperture 401 of the mask 400. This mask 400 may bea shadow mask or stencil mask and in this embodiment is illustratedupstream of the workpiece 300. The mask 400 blocks the first species 402from implanting the workpiece 300 except through the aperture 401. Theimplanted region 407 has a center and a periphery at each end of thedimension 408. The implanted region 407 may have a dose gradient orprofile such that it has a larger dose in the center than at theperiphery. The tails 410 illustrated in FIG. 8 may contribute to thesmaller dose at the periphery.

First, the dimension 403 between the mask 400 and the workpiece 300affects the size of any shoulders 409 or tails 410 implanted in theworkpiece 300. The dimension 403 may be increased until the spread ofthe species 402 in the ion beam increases and the larger dose region inthe center of the implanted region 407 decreases. For example, ifdimension 408 is approximately 500 μm, the dimension 403 may beapproximately 2-10 mm for an ion beam of species 402 having a beamspread of about one degree.

Second, the dimension 404 of the mask 400 affects the size of anyshoulders 409 or tails 410 implanted into the workpiece 300. Thisdimension 404 may be minimized to broaden the tails 410 and narrow theshoulders 409 or larger dose region in the center of the implantedregion 407.

Third, the angle 405 between the ion beam of species 402 and the mask400 affects the size of any tails 410. In FIG. 8, the angle 405 is about90°. A smaller angle 405 enables a wider tail 410 to be implanted intothe workpiece 300. However, only one tail 410 on one side of theshoulder 409 or the larger dose region in the center of the implantedregion 407 will be increased by making the angle 405 smaller than 90°.

Fourth, the angle 406 representing the beam spread of the ion beam ofspecies 402 affects the size of any tails 410. This angle 406 can beadjusted such that beam current implanting the workpiece 300 at anglesfar from the mean are increased. An optical lens, such as a decelerationlens, may be used to focus or spread the ion beam. Space charge in theion beam of species 402 also affects the angle 406.

In one embodiment, a desired dose at the periphery of the implantedregion 407 or other implanted regions in the workpiece 300 isdetermined. The dimension 403 is adjusted so that the desired dose isimplanted into the workpiece. In another embodiment, the angle 405 isadjusted. This may change the angle between the ion beam of species 402and the surface of the workpiece 300. Thus, the implant may not beperformed perpendicular to this surface of the workpiece 300. In yetanother embodiment, the angle distribution of the ion beam of species402 (such as angle 406) or other ion beams or species is adjusted. Thismay provide a desired dose. Of course, other dimensions also can bechanged to obtain the desired dose. Such changes will affect the lateraldose profile formed by the implant.

In an alternate embodiment, the ion beam of species 402 or other ionbeams or species may be focused. The optics can be adjusted so that theion beam of the species 402 or other ion beams or species has tails ofthe desired shape. For example, the voltage, plasma sheath shape, ordistance between the plasma sheath and the workpiece can be adjusted.The bias to a focusing element or lens also can be changed. Focusing ofan ion beam may occur with or without a stencil or shadow mask.

In yet another embodiment, a plasma system that modifies a plasma sheathmay be used. Such a system can be enabled to form a profile with broadtails. Changing the shape of the plasma sheath will affect the anglespread of the ion beam of the species 402 or other ion beams or species.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Thus, such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Furthermore, although the present disclosure hasbeen described herein in the context of a particular implementation in aparticular environment for a particular purpose, those of ordinary skillin the art will recognize that its usefulness is not limited thereto andthat the present disclosure may be beneficially implemented in anynumber of environments for any number of purposes. Accordingly, theclaims set forth below should be construed in view of the full breadthand spirit of the present disclosure as described herein.

What is claimed is:
 1. A method of implanting a workpiece comprising:implanting an entire surface of said workpiece with a first dopant; andimplanting said workpiece with a second dopant in a second region thatis a fraction of said surface, said second region having a center and aperiphery, wherein said second region has a larger dose at said centerthan at said periphery.
 2. The method of claim 9, wherein said workpieceis a solar cell.
 3. The method of claim 9, wherein one of said firstdopant and said second dopant is a p-type dopant and the other is ann-type dopant.
 4. The method of claim 9, further comprising implantingsaid surface of said workpiece with said first dopant in a first regionthat is a fraction of said surface, said first region having a centerand a periphery, wherein said first region has a larger dose at saidcenter than at said periphery, and wherein said periphery of said firstregion and said periphery of said second region at least partiallyoverlap.
 5. The method of claim 12, wherein said first region and saidsecond region extend to a depth deeper than a region formed by saidimplanting said entire surface of said workpiece with said first dopant.6. A solar cell comprising: a workpiece having a first surface; aplurality of n-type regions in said first surface, each of said n-typeregions having a center and a periphery, said center of said n-typeregion having a dose larger than in said periphery of said n-typeregion; and a plurality of p-type regions in said first surfacecomplementary to said plurality of said n-type regions, each of saidp-type regions having a center and a periphery, said center of saidp-type region having a dose larger than in said periphery of said p-typeregion, wherein said periphery of said p-type region and said peripheryof said n-type region at least partially overlap.
 7. The solar cell ofclaim 14, further comprising a junction at a location where said dose insaid periphery of said n-type region and said dose in said periphery ofsaid p-type region are equal.
 8. A solar cell comprising: a workpiecehaving a first surface, said first surface being doped to a firstconductivity and to a first depth; a plurality of second regions in saidfirst surface, each of said second regions being doped to a secondconductivity opposite said first conductivity, each of said secondregions having a center and a periphery, said center of said secondregion having a dose larger than in said periphery of said secondregion; and a space charge region between said plurality of said secondregions.
 9. The solar cell of claim 16, wherein an entirety of saidfirst surface is doped evenly to said first conductivity and to saidfirst depth.
 10. The solar cell of claim 16, wherein said firstconductivity and said second conductivity are selected from the groupconsisting of n-type and p-type.
 11. The solar cell of claim 16, whereina dopant profile of said periphery is configured to prevent shuntingacross said space charge region.
 12. The solar cell of claim 16, furthercomprising a plurality of first regions in said first surface, each ofsaid first regions being doped to said first conductivity, each of saidfirst regions having a center and a periphery, said center of said firstregion having a dose larger than in said periphery of said first region,wherein said periphery of said first region and said periphery of saidsecond region at least partially overlap, and wherein said first regionand said second region extend to a depth deeper than said first depth.